HttpWebRequest: Add Cookie to CookieContainer -> ArgumentException (Parametername: cookie.Domain)

CookieContainers can hold multiple cookies for different websites, therefor a label (the Domain) has to be provided to bind each cookie to each website. The Domain can be set when instantiating the individual cookies like so:

Cookie chocolateChip = new Cookie("CookieName", "CookieValue") { Domain = "DomainName" };

An easy way to grab the domain to is to make a Uri (if you aren’t using one already) that contains your target url and set the cookie’s domain using the Uri.Host property.

CookieContainer gaCookies = new CookieContainer();
Uri target = new Uri("http://www.google.com/");

gaCookies.Add(new Cookie("__utmc", "#########") { Domain = target.Host });